Structural Study of the Li0.5Na0.5MnFe2(PO4)3 and Li0.75Na0.25MnFe2(PO4)3 Alluaudite Phases and Their Electrochemical Properties As Positive Electrodes in Lithium Batteries

The alluaudite lithiated Phases Li0.5Na0.5MnFe2(PO4)(3) and Li0.75Na0.25MnFe2(PO4)(3) were prepared via a sol-gel synthesis, leading to powders with spongy characteristic's. The Rietveld refinement of the X-ray and neutron diffraction data coupled with ab initio calculations allowed us for the first time to accurately localize the lithium ions in the alluaudite structure. Actually, the lithium ions are localized in the A(1) and A(1)' sites of the tunnel. Mossbauer measurements showed the presence of some Fe2+ that decreased with increasing Li content. Neutron diffraction revealed the presence of a partial Mn/Fe exchange between the two transition metal sites that shows clearly that the oxidation state of the element is fixed by the type of occupied site. The electrochemical properties of the two phases were studied as positive electrodes in lithium batteries in the 4.5-1.5 V potential window, but they exhibit smaller electrochemical reversible capacity compared with the non-lithiated NaMnFe2(PO4)(3). The possibility of Na+/Li+ ion deintercalation from (Na,Li)MnFe2(PO4)(3) was also investigated by DFT+U calculations.